Rain overflow basins are used in so-called mixed systems for drainage technology, for example, to achieve a buffer effect in the event of suddenly occurring precipitation events such as heavy rain. A drainage of large areas, for example, in the catchment area of cities or on freeways can thus occur. In mixed systems, in particular dirty water, external water, and rainwater is drained in mixed form (as so-called mixed water) in a common line.
Rain overflow basins of the type in question can be embodied as catch basins, pass-through basins, or composite basins. Catch basins have the task of storing the first discharge (flushing surge) and supplying it in a throttled manner to the sewer system or a wastewater treatment facility. Pass-through basins have the task of storing mixed water, supplying it in a throttled manner to the sewer system or a wastewater treatment facility, and subjecting it to mechanical purification before the discharge via a clear water spillway, for example, by way of sedimentation or coarse material retention. Composite basins are a combination of catch basins and pass-through basins.
A rain overflow basin of the type in question generally has an inflow and outflow, each for connection to a sewer system, and a basin chamber, which is, for example, a storage chamber or a sedimentation chamber depending on the embodiment and function. The flow direction of the water generally occurs from the inflow to the outflow. A throttle structure having a throttle element, which is used for throttling the water flow, is often connected downstream of the outflow.
In general, rain overflow structures have a basin spillway, which is arranged before the basin chamber and starts after the filling of the basin chamber. The water of the basin spillway is discharged, for example, via an outlet or a channel (discharge channel), for example, into an outlet structure, a receiving water (for example, a body of water), or the like. The basin spillway is generally designed as a fixed threshold (spreading weir) having lateral incident flow, for example, having a spillway on one or both sides. Furthermore, an additional clear water spillway can be connected to the basin chamber, via which the mechanically clarified water can be discharged either in a throttled or unthrottled manner.
The rain overflow basin can be arranged in relation to the sewer system in the main connection, in the secondary connection, or in the false secondary connection. In the main connection, basin chamber and sewer system are hydraulically coupled both during the filling and also during the emptying, i.e., the basin chamber and the sewer system fill and empty simultaneously. In the secondary connection, basin chamber and sewer system are hydraulically decoupled by a partition structure, i.e., the basin chamber fills and empties later than the sewer system. In the false secondary connection, no unambiguous assignment to the main connection or secondary connection is possible, since sometimes a main connection and sometimes a secondary connection is provided during the filling and emptying.
In the event of mixed water introductions, in general the introduction of coarse materials or other unsightly material has to be restricted to an acceptable amount (cf., for example, DIN EN 752). For this purpose, rain overflow basins are to be equipped with rake or screen systems, which can be mechanically cleaned or are automatically cleaned by cleaning devices. In this case, rakes and fine screens having slot and hole widths of 4-8 mm have proven themselves. They are generally attached at or on the threshold, via which, for example, discharge takes place to the discharge channel, body of water, or the like. After a strong discharge procedure, in general the complete clogging of rakes and screens which are not cleaned by machine is to be expected, i.e., manual cleaning of the screens and rakes is necessary. Furthermore, the rakes and screens can be operated so that in the through-flow region, a filter layer builds up from the accumulating floating and suspended materials, which acts as an additional filter.
The retrofitting of existing rain overflow basins with such screens and rakes is often problematic since, for example, the nonreturn height can change or the installation of the purification device can be hampered or cannot be performed at all because of the dimensions of the weir threshold to the basin spillway.